1. Field of the Invention
This invention relates to thermoelectric devices. Particularly, this invention relates to sublimation suppression of Zintl-based thermoelectric power generation devices.
2. Description of the Related Art
Thermoelectric materials exhibit the property of producing an electric voltage from an applied temperature differential across the material, the so-called thermoelectric effect of Peltier-Seebeck effect. Accordingly, such materials may be used in thermoelectric devices to generate electrical power from a temperature differential. Such thermoelectric generators have been used to convert heat directly to electrical power for applications including isolated facilities or space applications. Depending upon the application, the applied heat may be naturally available or generated, e.g. by burning fuel or from a decaying radioisotope.
Previously, thermoelectric power generation for deep space applications have employed SiGe thermoelectric materials generating electric power using a decaying radioisotope, e.g. plutonium 238, as a heat source, in a radioisotope thermoelectric generator (RTG). The fuel source and solid state nature of the devices afford exceptional service life and reliability, paramount considerations in space applications which offset the relatively low efficiency of such devices. Many working RTG devices for space applications have been developed and successfully employed. See e.g. Winter et al., “The Design of a Nuclear Power Supply with a 50 Year Life Expectancy: The JPL Voyager's SiGe MHW RTG,” IEEE AES Systems Magazine, April 2000, pp. 5-12; and U.S. Pat. No. 3,822,152, issued Jul. 2, 1974 to Kot, which are incorporated by reference herein.
Recent focus on environmental and conservation issues has resulted in renewed interest in thermoelectric materials and devices. Zintl materials in particular have been studied for thermoelectric applications. A particular Zintl compound, Yb14MnSb11, has shown exceptional promise for thermoelectric power generation applications. See e.g. Brown et al., “Yb14MnSb11: New High Efficiency Thermoelectric Materials for Power Generation,” Chem. Mater., 18, 2006, 1873-1877, which is incorporated by reference herein. However, defining the properties of a particular material are only a first step in the development of a practical thermoelectric power generation device using that material.
SiGe has been well studied as a thermoelectric material as a result of previous RTG development. See e.g., Rowe, “Recent Advanced in Silicon-Germanium Alloy Technology and an Assessment of the Problems of Building the Modules for a Radioisotope Thermoelectric Generator,” Journal of Power Sources, 19 (1987), pp. 247-259; and “Silicon Germanium Thermoelectric Materials and Module Development Program,” ALO (2510)-T1, AEC Research and Development Rep, Cat. UC33, TID 4500, which are incorporated by reference herein. However, although the general configurations of previously developed SiGe thermoelectric power generation devices may be applicable, there are differences in the physical properties of Zintl materials and SiGe that demand new solutions in the development of a practical thermoelectric power generation devices using Zintl materials; the solutions for SiGe thermoelectric materials cannot be readily applied to Zintl thermoelectric materials.
At very high temperatures, e.g. 1,000 K, sublimation of the thermoelectric material can be quite high. Without suppressing sublimation of the thermoelectric material, the service life of a thermoelectric power generation device may be limited to only a few weeks. This may be further exacerbated in low pressure or vacuum environments such as in space applications. In the case of previously developed SiGe thermoelectric power generation devices, a coating of Si3N4 has been found as the preferred solution to suppress sublimation. However, Si3N4 is ineffective as a sublimation suppressor for Zintl materials, particularly Yb14MnSb11, because SiGe is very different from Zintl. Sublimation species and phenomena are different in different thermoelectric materials. Accordingly, different materials require different sublimation barriers. So, although a Si3N4 layer is suitable for SiGe, it is not going to work for Zintl. In another example, PbTe and Tellurium-Antimony-Germanium-Silver (TAGS), used for the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG), employs Ar gas as a sublimation suppression barrier because no coating option has been found to work with PbTe and TAGS. Thus, different thermoelectric materials require different sublimation suppression barrier solutions.
In view of the foregoing, there is a need in the art for apparatuses and methods for improved sublimation suppression in thermoelectric devices using Zintl materials such as Yb14MnSb11. There is particularly a need for such apparatuses and methods in Zintl-based thermoelectric devices operating at high temperatures, e.g. around or above 1,000 K. There is a need for such apparatuses and methods to extend the service life of such thermoelectric devices. There is a need for such apparatuses and methods to operate for such thermoelectric devices in space applications. These and other needs are met by embodiments of the present invention as detailed hereafter.